The present invention relates to a power transmission that transmits the driving force of an engine comprising a plurality of cylinders, through a ratio-change mechanism to drive wheels. Furthermore, the present invention relates particularly to a power transmission whose ratio-change mechanism is controlled by a line pressure which is set in a characteristic way.
Many automobiles are equipped with a power transmission that comprises such a ratio-change mechanism, and generally, the operation of such a ratio-change mechanism is controlled by means of hydraulic pressure. In this arrangement, the pressure of oil being supplied is set at a predetermined pressure (line pressure). For example, Japanese Laid-Open Patent Publication No. S60(1985)-256662 discloses a controller for a belt-type continuously variable transmission, to control the thrusts pushing the belt in the variable widths of the pulleys. In this case, a line pressure is supplied to the hydraulic cylinders used for adjusting the widths of the pulleys, and the line pressure is controlled in correspondence to the output torque of the engine, which torque is calculated from the rotational speed and suction pressure of the engine.
If the line pressure is controlled in correspondence to the output torque of the engine, then the engaging capacity of, for example, a clutch whose operation for the ratio-change mechanism is controlled by the line pressure can be set to a minimum value necessary for transmitting the output torque of the engine to the drive wheels. Therefore, the energy spent by the engine for generating the line pressure can be minimized to improve fuel efficiency. If the engaging capacity of the clutch is set to a minimum value necessary for transmitting the output torque of the engine in this way, then when the output of the engine or the load of the vehicle in travel motion changes abruptly (for example, the accelerator pedal is operated abruptly, or the vehicle is driven over a road-side step), the clutch can slip and prevent the torque being transmitted from changing abruptly. As a result, the vehicle can be improved in maneuverability and driving performance. Furthermore, in the belt-type continuously variable transmission controlled in this way, because the pressure acting on the belt is minimized, the durability of the belt itself can be improved. With such an arrangement, it is also possible to further miniaturize the ratio-change mechanism in a compact design.
Recently, for the purpose of improving fuel economy, vehicles are developed to be equipped with an idling elimination control, in which the operation of the engine is stopped when the vehicle comes into a halt, or with a partial cylinder operation control, in which, under certain driving conditions, some of the cylinders are disengaged from the operation of the engine. In comparison with an all cylinder operation mode, where all the cylinders are used for the operation of the engine, in a partial cylinder operation mode, the output of the engine is smaller because only some of the cylinders are employed for the operation of the engine. Therefore, for example, Japanese Laid-Open Patent Publication No. S59(1984)-13154 discloses a control for increasing the speed change ratio of the ratio-change mechanism used with such an engine which is equipped with disengageable cylinders when the engine is in a partial cylinder operation mode. However, the operation of the engine in a partial cylinder operation mode can occur not only for fuel saving, where the operation of the engine with some cylinders only is intentional, but also accidentally if the controller of the engine operation fails or breaks down and prevents some cylinders from participating in the engine operation.
Furthermore, while the vehicle is cruising, when the accelerator pedal is released, an engine brake is generated, and this brake torque is transmitted through the ratio-change mechanism to the wheels, decelerating the vehicle. For such a deceleration of the vehicle, it is preferable that the control parameters, for example, the line pressure, the transmission-torque capacity of the clutch, and the speed ratio change characteristics of the transmission be determined in correspondence to the engine brake torque (this term is also referred to as xe2x80x9cengine friction torquexe2x80x9d). Also, it is preferable that the engaging capacities of the clutch, etc. used in the ratio-change mechanism be adjustable appropriately, and that the belt-thrusting pressure and the speed ratio change characteristics of the continuously variable ratio-change mechanism be optimized accordingly. Here, as the engine brake torque differs depending on whether the engine is in an all cylinder operation mode or in a partial cylinder operation mode, the determination process of the control parameters should include such differences into consideration.
Nowadays, there is another type of power transmission in which an electrical motor generator is provided in connection to the output shaft of the engine. This motor generator assists the driving force of the engine as an electrical motor and regenerates energy as an electrical generator when the vehicle is in a deceleration. In such a power transmission, as the regenerating of energy is performed by the electrical motor generator during the deceleration, the torque transmitted through the ratio-change mechanism is determinable from the engine friction torque and the driving torque of the electrical motor generator. Therefore, the control parameters should be determined appropriately in correspondence to the torque values.
To solve the above mentioned problems, it is an object of the present invention to provide a power transmission which is equipped with an engine capable of operating in a partial cylinder operation mode and which enables appropriate determination of control parameters for a deceleration (the control parameters are, for example, the line pressure, the transmission-torque capacity, and the speed ratio change characteristic of the transmission).
Another object of the present invention is to provide a power transmission which is equipped with an energy-regenerating device and which enables appropriate determination of control parameters for a deceleration.
To achieve these objectives, the present invention provides a power transmission that comprises an engine, a ratio-change mechanism (for example, the belt-type continuously variable transmission CVT described in the following embodiment), a parameter-setting unit (for example, the control valve CV described in the following embodiment), and decelerating torque calculation means (for example, the electrical control unit ECU performing control steps S7 and S29 described in the following embodiment). The engine has a plurality of cylinders, and the ratio-change mechanism transmits the rotational driving force from the engine with a rotational speed change. The parameter-setting unit variably sets control parameters (for example, the line pressure, clutch transmission-torque capacity and ratio-change characteristic of the transmission) for controlling the operation of the ratio-change mechanism, and the decelerating torque calculation means calculates the decelerating torque (output shaft torque) of the engine when the engine is decelerating in a partial cylinder operation mode. While a vehicle equipped with this power transmission is decelerating with the engine operating in a partial cylinder operation mode, the parameter-setting unit sets the control parameters in correspondence to the decelerating torque of the engine calculated by the decelerating torque calculation means.
While the vehicle equipped with the power transmission is decelerating with the engine operating in a partial cylinder operation mode, the force of the engine brake gained from the cylinders not in operation differs from that gained from the cylinders in operation. Therefore, according to a feature of the present invention, the decelerating torque (engine friction torque or engine brake torque) of the engine operating in a partial cylinder operation mode is calculated by the decelerating torque calculation means, and in correspondence to this calculated decelerating torque of the engine, the control parameters, i.e., the clutch transmission-torque capacity, the line pressure, the speed-ratio change characteristic, etc. of the transmission, are set by the parameter-setting unit appropriately for an engine friction torque that is actually generated in this partial cylinder operation mode. Therefore, the engaging capacity of the clutch, etc. of the ratio-change mechanism can be set to a minimum value required for the transmission of the engine friction torque actually generated. This is advantageous in a case where the transmission torque changes abruptly during a deceleration, because the clutch, etc. can slip so as to prevent or restrain the transmission of this abrupt torque change to the drive wheels. Also, this feature of the present invention can be applied to adjust the pressing force of the belt of the ratio-change mechanism to a minimum value necessary so as to improve the durability of the belt and to further miniaturize the construction of the continuously variable ratio-change mechanism.
This power transmission can further comprise an energy-regenerating device (for example, the electrical motor generator M described in the following embodiment), which regenerates and stores energy from the kinetic energy of the vehicle in the deceleration. This energy-regenerating device is connected to the output shaft of the engine. In this case, the power transmission is preferably arranged to operate in such a way that while the vehicle is decelerating with the engine operating in a partial cylinder operation mode, the parameter-setting unit sets the control parameters in correspondence to the decelerating torque of the engine calculated by the decelerating torque calculation means and to the energy-regenerating condition of the energy-regenerating device.
When the power transmission is designed to regenerate energy by the energy-regenerating device during the deceleration of the vehicle, the torque being transmitted from the drive wheels through the ratio-change mechanism to the output shaft of the engine must counteract rotationally the torque of the energy-regenerating device in addition to the engine friction torque. If the clutch capacity, etc. of the transmission were determined in correspondence only to the engine friction torque while energy regenerating is being performed, then the clutch would slip with little rotation being transmitted to the energy-regenerating device. This would result in a greatly reduced efficiency of energy regenerating. Therefore, according to the present invention, the control parameters such as the clutch transmission-torque capacity and the line pressure are set by the parameter-setting unit in correspondence to the decelerating torque of the engine calculated by the decelerating torque calculation means and to the energy-regenerating condition of the energy-regenerating device. In this way, the control parameters are set, so that the rotational driving force from the wheels during the deceleration can be transmitted to the energy-regenerating device without any slip in the clutch, etc. of the ratio-change mechanism. As a result, the energy-regenerating device is operated efficiently, i.e., with no decrease in the energy regenerating efficiency.
Furthermore, if the ratio-change control characteristic of the transmission is also set in correspondence to the decelerating torque calculated accurately, then the ratio-change control characteristic can be set also accurately. Especially, if the inclination of the road where the vehicle is travelling is calculated from the decelerating torque calculated as described above and from the driving resistance, acceleration resistance, etc. of the vehicle, then the inclination of the road can be determined accurately, and this determined inclination of the road can be applied to the arrangement of a speed ratio control map which corresponds accurately to the inclination.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.